The present invention relates to a method for manufacturing a flame-retardant insulated wire and cable for use in nuclear power stations, which has excellent mechanical properties and environmental resistance.
An insulated wire and cable for use in nuclear power stations must satisfy strict requirements. That is, the cable is first subjected to a series of environmental tests such as aging, irradiation, and simulated loss of coolant accident (LOCA). Then, the cable must also pass a withstand voltage test. Moreover, the wire and cable must also pass a vertical tray flame test of cable and a vertical flame test of wire as defined by IEEE Std., 383 (1974).
In the manufacture of a flame-retardant insulated wire and cable, a thermoplastic resin or rubber composition containing hexabromobenzene, decabromodiphenyl ether, perchloropentacyclodecane or the like is known to be used as the rubber or plastic composition for insulation or jacket. In order to improve heat resistance, a method is known for performing crosslinking by a peroxide or by an electron beam.
However, an insulated layer and jacketed layer containing a flame retardant has inferior mechanical properties after irradiation in such environmental tests as described above. For example, an insulated wire, when wound around a mandrel having the same diameter, cracks in its insulating layer.
The present inventors have previously found that a rubber or plastic composition containing a halogenated acenaphthylene or a condensate thereof exhibits excellent environmental resistances such as flame retardancy and irradiation resistance. The present inventors have filed a patent application based on this finding (U.S. Pat. No. 4,373,046).
The present inventors have continued their studies to develop flame-retardant, crosslinked rubber or plastic insulated wires and cables using such a flame-retardant rubber or plastic composition. As a result of such studies, it has been found that a flame-resistant, crosslinked rubber or plastic insulated wire and cable manufactured according to the above-mentioned patent encounters a subsequent problem if it is subjected to the environmental tests required for use in nuclear power stations. More specifically, the present inventors prepared flame-resistant, crosslinked rubber or plastic insulated wires and cables by performing a most conventional crosslinking method, i.e., steam continuous crosslinking of a flame-retardant rubber or plastic composition containing the halogenated acenaphthylene or a condensate thereof as a flame retardant and an organic peroxide as a crosslinking agent, so as to obtain an insulation with excellent heat resistance and mechanical properties. When these wires and cables were subjected to the environmental tests required for use in nuclear power stations, they did not pass the subsequent withstand voltage test since the dielectric withstand characteristics were abruptly degraded after the environmental tests.
Generally known methods for improving water resistance of insulated wires and cables, are to use a polymer having a high water resistance, to improve the crosslinking density of the resin, to add a hydrophobic treatment agent such as a silicone coupling agent to the resin, and so on. Although each of these methods provides some slight improvement in water resistance, the wires and cables prepared by these methods did not pass the withstand voltage test.
However, the rubber or plastic composition itself, as a major constituent of the insulation, has characteristics which enable it to pass the withstand voltage test even if the wire and cable has degraded dielectric withstand characteristics following the environmental tests. Thus, it is considered that such degradation in dielectric withstand characteristics is attributable to the halogenated acenaphthylene or a condensate thereof used as a flame retardant. It was also found that a flame-retardant, crosslinked rubber or plastic insulated wire and cable containing a halogenated acenaphthylene or a condensate thereof has a problem of swelling which results in a significant increase in the outer diameter of the cable after the environmental tests. In order to provide a solution to the above-mentioned problem, the present inventors have made further studies. As a result of such studies, they have reached the conclusion that the degradation in the dielectric withstand characteristics of the wire and cable is due to some reaction during the environmental tests between a small amount of impurities contained in the halogenated acenaphthylene and steam to which the cable is exposed.
Further, it has been found that this problem can be solved by eliminating moisture from a heating atmosphere for crosslinking an insulating layer consisting of the rubber or plastic composition. The present invention has been established by this finding.